Semiconductor device

ABSTRACT

In order to supply a semiconductor device having high-reliability, there are used a first capacitor electrode, a capacitor insulating film formed in contact with the first capacitor electrode and mainly composed of titanium oxide, and a second capacitor electrode formed in contact with the capacitor insulating film, and there is used a conductive oxide film mainly composed of ruthenium oxide or iridium oxide for the first capacitor electrode and the second capacitor electrode. Alternatively, there is used a gate insulating film having a titanium silicate film and titanium oxide which suppress leakage current.

This application is a divisional application of U.S. Application Ser.No. 10/479,703, filed Dec. 5, 2003, which is a National Stage ofInternational Application No. PCT/JP02/05478, filed Jun. 4, 2002, theentire contents of which are incorporated herein by reference

TECHNICAL FIELD

The present invention relates to a semiconductor device.

BACKGROUND ART

Recently, the area of a memory capacitor has been reduced and theabsolute value of capacitance has been also reduced in accordance withminiaturization of a semiconductor device. The capacitance C, forexample in the case of a parallel-plate capacitor structure, isdetermined by C=ε·S/d, wherein ε means the dielectric constant of acapacitor insulating film, S means the area of an electrode, and d meansthe film thickness (distance between electrodes) of a dielectric. Inorder to assure a capacitance value without increasing the area S of anelectrode used for a capacitor element for information accumulation, itis necessary to use a capacitor insulating film material having a highdielectric constant ε or to decrease the film thickness d of a capacitorinsulating film.

Heretofore, a silicon oxide film has been used as a capacitor insulatingfilm, and high integration has been carried out by decreasing thethickness of this film. However, in a highly integrated memory of notless than 256 megabits, reduction of the film thickness has been put tothe limit, and, therefore, there has been introduced a capacitorinsulating film material such as tantalum oxide that has a higherdielectric constant ε than silicon oxide. Furthermore, in a DRAM(Dynamic Random Access Memory) of not less than 1 Gbit, there has beenconsidered use of a high dielectric constant material of bariumstrontium titanate (Ba_(x)Sr_(y)Ti_(s)O_(t):BST) as disclosed in, forexample, JP-A-9-186299. The similar problem is applicable to not only ahighly integrated memory but, also, with regard to a condenser used forvarious electronic circuits requiring miniaturization. For example, asdisclosed in JP-A-10-41467, there has been considered use of titaniumoxide having a high dielectric constant as a capacitor insulating filmmaterial for a condenser.

SUMMARY OF THE INVENTION

However, when titanium oxide was used as a capacitor insulating film andplatinum was used as an electrode material as disclosed inJP-A-10-41467, the dielectric constant was found not to be stable insome cases. In consideration of various contributing factors includingsuch cases, there is desired a semiconductor device having highreliability.

Thus, an objective of the present invention resides in providing asemiconductor device having high reliability.

In order to resolve the above problem, semiconductor devices accordingto the present invention include a construction in accordance with thefollowing discussion.

Specific examples are described below.

In the first place, by providing a thin film capacitor having highreliability, there can be provided a system-in-package having highreliability. Thereby, there can be provided a semiconductor devicehaving high reliability.

That is, said problem can be resolved by a thin film capacitor, asystem-in-package, and a semiconductor device having the followingconstitutions.

(1) A thin film capacitor comprising a first capacitor electrode, acapacitor insulating film formed in contact with the first capacitorelectrode, and a second capacitor electrode formed in contact with thecapacitor insulating film, wherein said capacitor insulating film iscomprised of mainly titanium oxide, and said first capacitor electrodeand said second capacitor electrode use conductive oxide filmscomprising mainly ruthenium oxide or iridium oxide.

In this connection, it is desirable that said capacitor insulating filmand said conductive oxide film have a film thickness of not less than0.9 nm and that said titanium oxide consists of crystals of rutilestructure.

(2) A system-in-package comprising a substrate and a circuit in which aLSI (Large Scale Integration Device), a condenser and a resistance areconnected by wiring divided with an insulating layer on one main surfaceside of said substrate, wherein said condenser comprises a firstcapacitor electrode, a capacitor insulating film formed in contact withthe first capacitor electrode, and a second capacitor electrode formedin contact with the capacitor insulating film, and wherein saidcapacitor insulating film comprises mainly titanium oxide and said firstcapacitor electrode and second capacitor electrode use conductive oxidefilms comprising mainly ruthenium oxide or iridium oxide.

In this connection, it is desirable that said capacitor insulating filmand said conductive oxide film have a film thickness of not less than0.9 nm and that said titanium oxide consists of crystals of rutilestructure.

(3) A semiconductor device comprising a semiconductor substrate, a firstcapacitor electrode formed on one main surface side of saidsemiconductor substrate, a capacitor insulating film formed in contactwith the first capacitor electrode, and a second capacitor electrodeformed in contact with the capacitor insulating film, wherein saidcapacitor insulating film comprises mainly titanium oxide, and saidfirst capacitor electrode and said second capacitor electrode useconductive oxide films comprising mainly ruthenium oxide or iridiumoxide.

In this connection, it is desirable that said capacitor insulating filmand said conductive oxide film have a film thickness of not less than0.9 nm and that said titanium oxide consists of crystals of rutilestructure. Herein, “comprise mainly” (or “consist mainly of”) means“contain at least 70 at. %”.

Secondly, by providing a semiconductor device having improved gateelectrode structure such as a gate insulating film that can suppressleakage current effectively, there can be provided a semiconductordevice having high reliability.

Recently, in a semiconductor device provided with plural MOS (MetalOxide Semiconductor) transistors each of which has a gate insulatingfilm present between a semiconductor substrate and a gate electrode,reduction in thickness of the gate insulating film has been required andan oxide film of not more than 3.0 nm in thickness has been used inaccordance with miniaturization of a semiconductor device. Whenthickness of the insulating film is reduced to 3.0 nm or less, therearises a problem in which direct tunnel current (hereinafter referred toas DT current) becomes too large to be ignored, leakage currentincreases, and electric power consumption increases. Thus, by using as agate insulating film titanium oxide or the like, having a higherdielectric constant than that of silicon oxide, which leads toimprovement in the dielectric characteristics, and, at the same time,increase the thickness of the gate insulating film, a tendency forincrease in DT current can be suppressed. For example, when relativedielectric constants of titanium oxide and silicon oxide arerespectively 60 and 4.0, it follows that a titanium oxide film of 30 nmin thickness has dielectric characteristics equivalent to those of asilicon oxide film of 2 nm in thickness. In such case, the titaniumoxide film of 30 nm in thickness is said to have a silicon-oxideequivalent thickness of 2 nm. On the other hand, the factual thicknessof 30 nm is called a physical film thickness or factual film thickness.

On the other hand, when a titanium oxide film is formed on a siliconsubstrate, in some cases oxygen atoms in the titanium oxide film diffuseto the silicon substrate side to form silicon oxide at the interfacebetween the titanium oxide film and the silicon substrate. The formationof silicon oxide increases the equivalent thickness of a gate insulatingfilm. For example, when silicon oxide is formed at said interface in athickness of not less than 1 nm, it becomes impossible to have theequivalent thickness of a gate insulating film in the range of not morethan 1 nm.

Thus, in order to prevent formation of silicon oxide at said interface,there is contrived a method of forming a silicon nitride film between atitanium oxide film and a silicon substrate (for example, seeJP-A-2000-58831). Formation of silicon oxide at said interface can besuppressed by forming a silicon nitride film between a titanium oxidefilm and a silicon substrate. However, silicon nitride has only arelative dielectric constant of about 7.8, and when a gate insulatingfilm is allowed to have a silicon-oxide equivalent thickness of not morethan 1 nm, the factual thickness thereof is decreased and leakagecurrent by direct tunnel is increased. Hence, there is a possibilitythat leakage current would surpass the acceptable value.

The possibility that leakage current is increased and would surpass theacceptable value reduces yield of products and causes reduction inreliability of the products.

Thus, in order to supply a semiconductor device having high reliability,there is produced a semiconductor device provided with plural MOStransistors each of which has a gate insulating film constituted so asto contain a titanium oxide film, wherein formation of silicon oxide issuppressed at the interface between the titanium oxide film and asilicon substrate, and wherein the gate insulating film is allowed tohave a silicon-oxide equivalent thickness of not more than 1 nm.

Alternatively, there is produced a semiconductor device wherein leakagecurrent flowing through the gate insulating film can be suppressed to alow extent.

Furthermore, there is produced a semiconductor device having a highyield.

The inventors carried out experiments and calculations with variousmaterials and, as a result, have found that when a gate insulating filmis constituted by forming a titanium silicate film on the surface of asilicon substrate and forming thereon a titanium oxide film, diffusionof oxygen atoms from the titanium oxide film to the silicon substratecan be prevented, and leakage current can be reduced effectively becausethe relative dielectric constant of titanium silicate is larger thanthat of silicon nitride.

(4) The present invention resolving the above problems provides for asemiconductor device comprising plural MOS transistors each of which hasa gate insulating film disposed between a semiconductor substrate and agate electrode, characterized by the fact that said gate insulating filmhas a laminated structure containing a titanium silicate film formed onthe semiconductor substrate side and a titanium oxide film formed on thegate electrode side.

In this case, it is desirable that the silicon-oxide equivalentthickness of said gate insulating film which is obtained from dielectriccharacteristics, is not more than 1.0 nm, and that said titaniumsilicate film has a factual thickness of not less than 1.0 nm but notmore than 3.2 nm.

Actually, it is desirable that a factual thickness, T₂, of said titaniumsilicate film is formed to fall within a range represented by1.0 (nm)≦T ₂≦5T _(eff)−1.8 (nm)wherein T₂ represents the factual thickness of said titanium silicatefilm and T_(eff) represents the silicon-oxide equivalent thickness ofsaid gate insulating film.

(5) Furthermore, in preparing a semiconductor device comprising pluralMOS transistors each of which has a gate insulating film present betweena semiconductor substrate and a gate electrode, the gate insulating filmis formed by a step including a procedure of forming a titanium silicatefilm on the semiconductor substrate and a procedure of forming atitanium oxide film on the titanium silicate film.

The titanium silicate film can be formed by either a method of forming atitanium film on the surface of a silicon substrate, heat-treating thetitanium film into a titanium silicide film, and oxidizing the titaniumsilicide film into the titanium silicate film, or a method of forming asilicon oxide film on the surface of the silicon substrate, forming atitanium film superposed on the silicon oxide film, and reacting the twoby heat treatment to form the titanium silicate film.

The semiconductor device of the present invention has a titaniumsilicate film at the interface between titanium oxide and a siliconsubstrate, and hence formation of a silicon oxide film having a lowrelative dielectric constant at said interface can be suppressed.Therefore, the silicon-oxide equivalent thickness of a gate insulatingfilm can be reduced.

Furthermore, the present semiconductor device has, as the gateinsulating film, titanium oxide which is a high dielectric constantmaterial, and the titanium silicate film having a relatively largedielectric constant. Hence, the factual thickness of the gate insulatingfilm can be increased and the silicon-oxide equivalent thickness thereofcan be reduced. Therefore, leakage current can be reduced.

Moreover, because a semiconductor device wherein leakage current isdifficult to flow can be obtained, there can be produced a semiconductordevice having high reliability and, also, having a high yield.

Other objects, characteristics and advantages of the present inventionwill be clear from the following description of the working embodimentsof the present invention relating to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is the sectional view of the main portion of a semiconductordevice according to a first example of the present invention.

FIG. 2 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 3 nm tocapacitor electrodes of 3 nm in thickness with regard to the presentinvention.

FIG. 3 shows diffusion constants at 600° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 3 nm tocapacitor electrodes of 3 nm in thickness with regard to the presentinvention.

FIG. 4 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of anatase structure having a thickness of 3 nm tocapacitor electrodes of 3 nm in thickness with regard to the presentinvention.

FIG. 5 shows diffusion constants at 600° C. of oxygen diffusing from atitanium oxide film of anatase structure having a thickness of 3 nm tocapacitor electrodes of 3 nm in thickness with regard to the presentinvention.

FIG. 6 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 0.9 nm tocapacitor electrodes of 0.9 nm in thickness with regard to the presentinvention.

FIG. 7 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of anatase structure having a thickness of 0.9 nm tocapacitor electrodes of 0.9 nm in thickness with regard to the presentinvention.

FIG. 8 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 0.9 nm tocapacitor electrodes of 0.8 nm in thickness with regard to the presentinvention.

FIG. 9 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 0.8 nm tocapacitor electrodes of 0.9 nm in thickness with regard to the presentinvention.

FIG. 10 is the sectional view of the main portion of a semiconductordevice according to a second example of the present invention.

FIG. 11 is the sectional view of the main portion of a semiconductordevice according to a third example of the present invention.

FIG. 12 is the sectional view of the main portion of a thin filmcapacitor according to a fourth example of the present invention.

FIG. 13 is the sectional view of the main portion of a thin filmcapacitor according to a fifth example of the present invention.

FIG. 14 is the sectional view of the main portion of a thin filmcapacitor according to a sixth example of the present invention.

FIG. 15 is the sectional view of the main portion of a thin filmcapacitor according to a seventh example of the present invention.

FIG. 16 is the sectional view of the main portion of a system-in-packageaccording to an eighth example of the present invention.

FIG. 17 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 30 nm tocapacitor electrodes of 3 nm in thickness with regard to the presentinvention.

FIG. 18 shows diffusion constants at 300° C. of oxygen diffusing from atitanium oxide film of rutile structure having a thickness of 35 nm tocapacitor electrodes of 3 nm in thickness with regard to the presentinvention.

FIG. 19 is the sectional view of the main portion of a semiconductordevice according to a ninth example of the present invention.

FIG. 20 is the plan view showing the main portion of the semiconductordevice of the example shown in FIG. 19.

FIG. 21 is the conceptual diagram showing energy bands of the gateelectrode, titanium oxide, titanium silicate and silicon substrate inthe example shown in FIG. 19.

FIG. 22 is the conceptual diagram showing energy bands of the gateelectrode, titanium oxide, titanium silicate and silicon substrate whenthe electric voltage, V, was applied to the gate electrode in theexample shown in FIG. 19.

FIG. 23 is the graph showing dependency of leakage current density onthe thickness and equivalent thickness of titanium silicate film whenthe relative dielectric constant of titanium silicate is 15, thesilicon-oxide equivalent thickness of the gate insulating film is 1.0nm, and the electric voltage applied to the gate insulating film is 1.0V in the ninth example of the present invention.

FIG. 24 is the graph showing dependency of leakage current density onthe thickness and equivalent thickness of titanium silicate film whenthe relative dielectric constant of titanium silicate is 20, thesilicon-oxide equivalent thickness of the gate insulating film is 1.0nm, and the electric voltage applied to the gate insulating film is 1.0V in the ninth example of the present invention.

FIG. 25 is the graph showing dependency of leakage current density onthe thickness and equivalent thickness of titanium silicate film whenthe relative dielectric constant of titanium silicate is 25, thesilicon-oxide equivalent thickness of the gate insulating film is 1.0nm, and the electric voltage applied to the gate insulating film is 1.0V in the ninth example of the present invention.

FIG. 26 is the graph showing dependency of leakage current density onthe thickness and equivalent thickness of titanium silicate film whenthe relative dielectric constant of titanium silicate is 30, thesilicon-oxide equivalent thickness of the gate insulating film is 1.0nm, and the electric voltage applied to the gate insulating film is 1.0V in the ninth example of the present invention.

FIG. 27 is the graph showing dependency of leakage current density onthe thickness and equivalent thickness of titanium silicate film whenthe relative dielectric constant of titanium silicate is 15, thesilicon-oxide equivalent thickness of the gate insulating film is 1.0nm, and the electric voltage applied to the gate insulating film is 0.5,0.7, and 1.0 V in the ninth example of the present invention.

FIG. 28 is the graph showing dependency of leakage current density onthe thickness and equivalent thickness of titanium silicate film whenthe relative dielectric constant of titanium silicate is 15, thesilicon-oxide equivalent thickness of the gate insulating film is 0.7nm, and the electric voltage applied to the gate insulating film is 0.5,0.7, and 1.0 V in the ninth example of the present invention.

FIG. 29 is the graph showing the desirable range of factual filmthickness of titanium silicate when the silicon-oxide equivalentthickness of the gate insulating film is 0.7 to 1.0 nm in the ninthexample of the present invention.

FIG. 30 is the sectional view showing an example wherein the gateelectrode has a two layer structure of tungsten nitride film andtungsten film in the example shown in FIG. 19.

FIGS. 31(A)-31(C) show sectional views for illustrating a phase in theprocess of producing the main portion of the semiconductor device shownin FIG. 19.

FIGS. 32(A)-32(C) show sectional views for illustrating a phase in theprocess of producing the main portion of the semiconductor device shownin FIG. 19, which follow FIGS. 31(A)-31(C).

FIGS. 33(A)-33(C) show sectional views for illustrating a phase in theprocess of producing the main portion of the semiconductor device shownin FIG. 19, which follow that of FIGS. 32(A)-32(C).

FIGS. 34(A)-34(D) show sectional views for illustrating an alternativeearly phase in the process of producing the main portion of thesemiconductor device shown in FIG. 19.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the mode for carrying out the present invention will bedescribed in detail. First, the sectional structure of the main portionof a DRAM (Dynamic Random Access Memory) memory cell which is the firstexample in the present invention, is shown in FIG. 1. As shown in FIG.1, the semiconductor device of the present example is provided with MOS(Metal Oxide Semiconductor) type transistors 2 formed on a siliconsubstrate 1 which is a semiconductor, and a memory capacitor 3 disposedabove the transistors. An insulating film 12 is a film for separatingelements. In this connection, structures of the present figure and theother figures are schematically shown in order to help in theunderstanding of the present examples including the circuits thereof andwhich are mere examples.

The MOS transistor 2 in the memory cell is constituted by a gateelectrode 5, a gate insulating film 6 and diffusion layers 7. The gateinsulating film 6 consists of, for example, silicon oxide film, siliconnitride film or a high dielectric constant film or a laminated structurethereof. The gate electrode 5 consists of, for example, polycrystallinesilicon film or metal thin film, conductive oxide film or metal silicidefilm or a laminated structure thereof. On the top and side walls of saidgate electrode 5, there is formed an insulating film 9 consisting of,for example, silicon oxide film. The one diffusion layer 7 of the MOStransistor for memory cell selection is connected to a bit-line 11through a plug 10. In the whole portion above the

MOS transistors, there is formed an insulating film 12 consisting of,for example, BPSG (Boron-doped Phospho Silicate Glass) film or SOG (SpinOn Glass) film, or silicon oxide or nitride film formed by chemicalvapor phase deposition method or sputtering method, or the like.

A memory capacitor 3 is formed on the insulating film 12 covering theMOS transistors. The memory capacitor 3 is connected to the otherdiffusion layer 8 of the MOS transistor for memory cell selectionthrough a plug 13 consisting of, for example, polycrystalline silicon ortungsten or the like. The memory capacitor 3 is constituted by alaminated structure of a conductive barrier film 14, a capacitor lowerelectrode 15, a capacitor insulating film 16 consisting mainly oftitanium oxide, and a capacitor upper electrode 17 in the order from thelowermost layer. This memory capacitor 3 is covered by an insulatingfilm 18. The conductive barrier film 14 consists of, for example,titanium, titanium nitride, tantalum, tantalum nitride or the like. Inthis connection, the conductive barrier film 14 may be absent, forexample, in the case where adhesion between the capacitor lowerelectrode 15 and the plug 13 is good and, furthermore, their counterdiffusion scarcely takes place.

The inventors have found that when as a material for the capacitor lowerelectrode 15 and the capacitor upper electrode 17 there is usedpolycrystalline silicon, tungsten, tungsten silicide, molybdenum,molybdenum silicide, ruthenium, iridium, platinum or the like, oxygendiffuses to the capacitor electrodes from the capacitor insulating film16 comprising mainly titanium oxide, and oxygen deficit is caused in thecapacitor insulating film 16, and, furthermore, the inventors have foundthat the dielectric constant is not stable because of this oxygendeficit. Moreover, the inventors carried out intense research in orderto obtain a means for suppressing diffusion of oxygen to a capacitorelectrode from a capacitor insulating film comprising mainly titaniumoxide, and, as a result, they have found that it is effective to useruthenium oxide or iridium oxide as a capacitor electrode material whichcontacts with titanium oxide. Thus, in the present example, a conductiveoxide film comprising mainly ruthenium oxide or iridium oxide is usedfor the capacitor lower electrode 15 and the capacitor upper electrode17 so that oxygen hardly diffuses to the electrodes from the capacitorinsulating film 16 comprising mainly titanium oxide. This conductiveoxide film is formed by use of, for example, chemical vapor phasedeposition method, sputtering method or the like.

With regard to diffusion of oxygen from titanium oxide to electrodes, bycomparing ruthenium oxide and iridium oxide used in the present examplewith polycrystalline silicon, tungsten, tungsten silicide, molybdenum,molybdenum silicide, ruthenium, iridium, and platinum, which have beenconsidered as a capacitor electrode material, the effect of the presentexample is illustrated as follows.

In order to explain the effect of the present example in detail, thereis shown an analytical example based on molecular dynamics simulation.The molecular dynamics simulation is a method of calculating a forceacting on each atom through interatomic potential, and calculatingposition of each atom at each time by resolving Newton's equation ofmotion on the basis of said force, as stated in, for example, Journal ofApplied Physics, Vol. 54 (issued in 1983), pages 4864-4878. In thisconnection, in the present example, the below-mentioned relation couldbe obtained by taking into account charge-transfer in said moleculardynamics method and calculating the interaction between differentelements.

The main effect of the present example is to suppress diffusion ofoxygen from a capacitor insulating film to capacitor electrodes.Diffusion of other elements is also suppressed, but herein the effect ofthe present example is illustrated by calculating diffusion constants ofoxygen, which diffuses to capacitor electrodes, and comparing thecalculation results. The method for calculating diffusion constants bythe molecular dynamics simulation is stated in, for example, PhysicalReview B, Vol. 29 (issued in 1984), pages 5363-5371.

First, the effect of the present example is shown by use of acalculation example in the case of using a laminated structure of acapacitor electrode having a film thickness of 3 nm and a capacitorinsulating film having a thickness of 3 nm. As the capacitor insulatingfilm, there was used a titanium oxide film of rutile structure oranatase structure, and as the capacitor electrode material there wereused polycrystalline silicon, tungsten, tungsten silicide, molybdenum,molybdenum silicide, ruthenium, iridium, and platinum, which have beenconsidered as a capacitor electrode, and ruthenium oxide and iridiumoxide used in the present example.

Calculation results of diffusion constants, when oxygen diffuses to theelectrodes from the titanium oxide film of rutile structure at 300° C.,are shown in FIG. 2. Moreover, diffusion constants at 600° C. are shownin FIG. 3.

When diffusion constants at 300° C. are not less than 10⁻²⁰ m²/s, muchoxygen deficit is formed in the capacitor insulating film. Hence, inorder to ensure the reliability of a semiconductor device such as shownin FIG. 1, it is preferable that diffusion constants at 300° C. are lessthan 10⁻²⁰ m²/s.

From these figures it is seen that when ruthenium oxide or iridium oxidewas used as an electrode in the cases of both 300° C. and 600° C.,smaller diffusion constants are shown as compared with the other cases.That is, when ruthenium oxide or iridium oxide was used as an electrode,it can be said that oxygen hardly diffuses to the electrode andreliability is high. FIG. 2 and FIG. 3 show calculation results in thecase of using titanium oxide of rutile structure, but calculationresults of diffusion constants in the case of using titanium oxidehaving anatase structure are as shown in FIG. 4 and FIG. 5. FIG. 4 andFIG. 5 show calculation results respectively at 300° C. and 600° C. Alsoin these figures, similarly to FIG. 2 and FIG. 3, when ruthenium oxideor iridium oxide was used as an electrode, smaller diffusion constantsare shown as compared with the other cases. When calculation results ofFIG. 4 and FIG. 5 are compared with those of FIG. 2 and FIG. 3, it isseen that diffusion constants in the case of using rutile structure aresmaller than in the case of anatase structure. Therefore, it is morepreferable to use titanium oxide of rutile structure as a capacitorinsulating film and use ruthenium oxide or iridium oxide as a capacitorelectrode. The titanium oxide of rutile structure is formed by a methodof film formation at a high temperature or film formation at a lowtemperature and the subsequent heat treatment as stated in, for example,IBM Journal of Research and Development, Vol. 43, No. 3 (issued in May1999), pages 383-391.

FIG. 2, FIG. 3, FIG. 4 and FIG. 5 show calculation results when 3 nm wasselected as the thickness of a capacitor electrode film and that of acapacitor insulating film, but in order to study dependency of diffusionconstant on film thickness, results obtained by changing thesethicknesses are shown hereinafter. When 0.9 nm was selected as both thethickness of a capacitor electrode film and that of a capacitorinsulating film, calculation results at 300° C. for rutile structure andanatase structure are shown respectively in FIG. 6 and FIG. 7.

From FIG. 6 and FIG. 7, similarly to the case of 3 nm film thickness,even when the film thickness is reduced to 0.9 nm, it is seen thatdiffusion constants for ruthenium oxide and iridium oxide are remarkablysmall as compared with those for the other materials. Though not shownin the figures, also, in the case of 600° C., there was obtained theresult that diffusion constants for ruthenium oxide and iridium oxideare remarkably small as compared with those for the other materials.

On the other hand, when 0.9 nm was retained as the thickness of acapacitor insulating film and 0.8 nm was selected as the thickness of acapacitor electrode film, calculation results for rutile structure at300° C. are shown in FIG. 8. In this case, when compared with FIG. 6 andFIG. 7, it is seen that diffusion constants for ruthenium oxide andiridium oxide are considerably greater than 10⁻²⁰ m²/s , in which theeffect of the present example is reduced. Therefore, it is morepreferable that the film thickness of ruthenium oxide or iridium oxideis not less than 0.9 nm. Next, when 0.9 nm was retained as the thicknessof a capacitor electrode film and 0.8 nm was selected as the thicknessof a capacitor insulating film, the calculation results for a rutilestructure at 300° C. are shown in FIG. 9.

Also in this case, when compared with FIG. 6 and FIG. 7, it is seen thatdiffusion constants for ruthenium oxide and iridium oxide are remarkablylarge and the effect of the present example is reduced. Therefore, it ismore preferable, also, that the film thickness of titanium oxide is 0.9nm or more.

The above phenomenon that diffusion constants become suddenly large whenthe thickness of an electrode or that of a capacitor insulating film is0.8 nm or less is considered to occur because of the following reason.The diameter of atoms is approximately 0.1 to 0.3 nm, and 0.8 nmcorresponds to the state wherein atoms stand in 4 to 8 lines. In thisstate, it is presumed that atoms in adjacent films commingle and wouldlead to a loss of film function.

FIG. 8 and FIG. 9 show results for rutile structure. Similarly, alsowith regard to anatase structure, there was obtained the result that afilm thickness of 0.9 nm or more is more preferable. The effect isreduced in a film thickness of 0.8 nm or less, because crystalstructures of ruthenium oxide, iridium oxide and titanium oxide becomeslightly unstable. In addition, though not shown in FIG. 2 to FIG. 7,when a material of low melting point such as gold or silver is used asan electrode material, diffusion constant of oxygen becomes larger thanin the case of using silicon as an electrode. Therefore, it is notpreferable either to use gold or silver as an electrode material whichcontacts with titanium oxide.

Next, the sectional structure of the main portion of a DRAM (DynamicRandom Access Memory) memory cell according to the second example of thepresent invention is shown in FIG. 10. The main difference from thefirst example resides in the point that the capacitor has not aparallel-plate structure but a rectangular structure. The use of a samenumber in FIG. 10 as in FIG. 1 means the same constituent component.This structure has the advantage that the capacitor has a largereffective area and a larger capacity. Also, in the present example, byuse of a capacitor electrode consisting mainly of ruthenium oxide oriridium oxide, the suppression effect of oxygen diffusion can beobtained similarly to that in the first example. In addition, thecapacitor may have a structure other than these structures.

Next, the sectional structure of the main portion of a DRAM (DynamicRandom Access Memory) memory cell according to the third example of thepresent invention is shown in FIG. 11. The main difference from thesecond example resides in the point that the capacitor electrode has adouble structure. That is, the capacitor lower electrode is constitutedby a conductive film 15 and a conductive film 19, and the capacitorupper electrode is constituted by a conductive film 17 and a conductivefilm 20. The use of a same number in FIG. 11 as in FIG. 10 means thesame constituent component. In the case of FIG. 11, conductive films 15and 17, which are in direct contact with the capacitor insulating film16, consist of a film consisting mainly of ruthenium oxide or iridiumoxide in order to suppress diffusion of oxygen from the capacitorinsulating film. Electrode films 19 and 20 which do not directly contactwith the capacitor insulating film, preferably consist of a materialhaving a lower electric resistance than that of ruthenium oxide oriridium oxide such as ruthenium, iridium, platinum, osmium, rhodium,palladium, tungsten, molybdenum, gold, silver, or the alloys or silicidecompounds thereof, or the like. The similar matter is applicable also tothe electrodes of the following examples. In addition, an electrodestructure is not limited to the structures shown herein, and otherlayers may be furthermore contained therein. Moreover, only the upperelectrode may be formed by plural layers and the lower layer maycomprise a single layer. On the contrary, only the lower electrode maybe formed by plural layers and the upper layer may comprise a singlelayer. The similar matter is applicable also to the electrodes of thefollowing examples.

The above examples relate to DRAM (Dynamic Random Access Memory), andfor products having a thin film capacitor containing a capacitorinsulating film comprising mainly titanium oxide there can be usedelectrodes comprising mainly ruthenium oxide or iridium oxide.

Next, a thin film capacitor according to the fourth example of thepresent invention is described by use of FIG. 12. In the presentexample, the thin film capacitor 102 is formed on a substrate 101consisting of, for example, a semiconductor material, a resin, a glassor the like. This thin film capacitor 102 consists of a conductivebarrier film 103, a capacitor lower electrode 104, a capacitorinsulating film 105 comprising mainly titanium oxide, and a capacitorupper electrode 106 in the order from the lowermost layer. Theconductive barrier film 103 consists of titanium, titanium nitride,tantalum, tantalum nitride or the like. As the main constituent materialof the capacitor lower electrode 104 and the capacitor upper electrode106, there is used ruthenium oxide or iridium oxide to which oxygenhardly diffuses from the capacitor insulating film 105. The thin filmcapacitor 102 is connected to a first layer wire 107 through a plug 108consisting of, for example, copper, tungsten or the like.

A second layer wire 109 is connected to the first layer wire 107 througha plug 110. In addition, though not shown in the figure, the secondlayer wire 109 and the capacitor upper electrode 106 are connected toanother wire. Barrier films 111, 112, 113, 114, 115, 116, 117, and 118are adjacent to the first layer wire 107, second layer wire 109, plug108 and plug 110. When the first layer wire 107, second layer wire 109,and plug 108 have excellent adhesion with an insulating film 119 and,furthermore, hardly cause interdiffusion with the insulating film 119,these barrier films may be absent. 120 and 121 are insulating films. Thethin film capacitor 102 is used, for example, as a filter for flowingonly alternating electric current of a specific range of frequency tothe second layer wire.

Subsequently, a thin film capacitor according to the fifth example inthe present invention is described by use of FIG. 13. The maindifference between the present example and the fourth example resides inthe point that, in the latter, the capacitor insulating film comprisingmainly titanium oxide is used as an insulating film between the firstlayer wire and second layer wire, and the structure of the presentexample is simple. In addition, the use of a same number in FIG. 13 asin FIG. 12 means the same constituent component. According to thepresent example, the number of film formation steps can be reduced.

Furthermore, the layout of a thin film capacitor is not limited to thatas above-described. For example, even the layout as shown in FIG. 14 isusable. The main difference between the sixth example and the fourthexample resides in the point that, in the latter, the thin filmcapacitor 102 is positioned at a layer between the second layer wire 109and a third layer wire 125. The use of a same number in FIG. 14 as inFIG. 12 means the same constituent components. In FIG. 14, further,reference numerals 123, 124, 126 and 127 are barrier films, 122 is aplug for interconnecting wires 109 and 125, and reference numerals 128and 129 are insulating films.

In addition, as a simpler example, there is cited the layout as shown inFIG. 15. The seventh example has the structure wherein a thin filmcapacitor 102 is formed on one main surface of a substrate 101comprising, for example, silicon or the like. The thin film capacitor102 comprises a conductive barrier film 103, a capacitor lower electrode104, a capacitor insulating film 105 comprising mainly titanium oxide,and a capacitor upper electrode 106 in the order from the lowermostlayer. The conductive barrier film 103 comprises, for example, titaniumnitride, titanium or the like. In addition, though not shown in thefigure, there may be present a single or plural other films between thebarrier film 103 and the substrate 101.

Subsequently, the main sectional view of a system-in-package accordingto the eighth example of the present invention is shown in FIG. 16. Thesystem-in-package is the one wherein a LSI (Large Scale IntegratedCircuit) and passive elements are integrated on a substrate, as statedin, for example, Nikkei Micro-device, March 2001, pages 114-123. In thepresent example, a LSI 218, a thin film capacitor 202, and a resistance226 are formed on a substrate 201 consisting of, for example, a resin,an organic material, a glass, silicon or the like. The thin filmcapacitor 202 consists of a capacitor lower electrode 203, a capacitorinsulating film 204 comprising mainly titanium oxide, and a capacitorupper electrode 205 in the order from the lowermost layer. As the mainconstituent material of the capacitor lower electrode 203 and thecapacitor upper electrode 205, there is used ruthenium oxide or iridiumoxide to which oxygen hardly diffuses from the capacitor insulating film204. The capacitor lower electrode 203 of this thin film capacitor 202is connected to a wire 206 interposed between barrier films 207 and 208.In this connection, when the wire 206 is comprised mainly of copper, itis preferable in the point of preventing the wire from peeling to useruthenium or iridium as the main constituent material of the barrierfilm 208. This is because the adhesion between copper and ruthenium andthat between copper and iridium are excellent. Furthermore, rutheniumand iridium have good adhesion, also, with ruthenium oxide or iridiumoxide, either of which may be used as the main constituent material ofthe capacitor lower electrode 203, and hence there is obtained astructure which is hard to delaminate. This can similarly be saidregarding the capacitor upper electrode 205. That is, the capacitorupper electrode 205 is connected to a wire 209 interposed betweenbarrier films 210 and 211, and hence it is preferable in the point ofpreventing the wire from peeling to use ruthenium or iridium as the mainconstituent material of the barrier film 210. In FIG. 16, referencenumerals 212, 215, 219, 222, 227, 230, 233 and 236 show wires comprisingmainly, for example, copper, and 213, 214, 216, 217, 220, 221, 223, 224,228, 229, 231, 232, 234, 235, 237 and 238 show barrier films comprisingmainly, for example ruthenium. In addition, 225 and 239 show aninsulating layer consisting of, for example, a resin or the like. Thethin film capacitor 202 is used, for example, as a filter for flowingonly alternating electric current of a specific range of frequency tothe resistance 226 and wire 227 among electric current flowing to thewire 222. In place of this thin film capacitor 202, there may be used athin film capacitor formed on a substrate as shown in the seventhexample.

In addition, though not shown in the figure, in some cases a memory chipsuch as ROM, RAM or the like may be provided in this system-in-package.There is the effect that since miniaturization of a capacitor ispossible, flexibility in the layout of the capacitor is increased indesigning complicated wiring.

Furthermore, in order to consider in more detail the dependency ofeffect on film thickness, similarly to that shown in FIG. 2 to FIG. 9,diffusion constants of oxygen when the thickness of the capacitorinsulating film was changed to 30 nm are shown in FIG. 17. Moreover,diffusion constants of oxygen when the thickness of the capacitorinsulating film was changed to 35 nm, are shown in FIG. 18. According toFIG. 17, when the thickness of the capacitor insulating film was changedto 30 nm, in the case of using electrode materials other than rutheniumoxide and iridium oxide, the diffusion constants at 300° C. become 10⁻²⁰m²/s or more. On the other hand, according to FIG. 18, when thethickness of the capacitor insulating film was changed to 35 nm, even inthe case of using electrode materials other than ruthenium oxide andiridium oxide, the diffusion constants at 300° C. become smaller than10⁻²⁰ m²/s. When diffusion constants at 300° C. are 10⁻²⁰ m²/s or more,much lack of oxygen is caused in the capacitor insulating film . Hence,in order to ensure reliability of a semiconductor device such as shownin FIG. 1, it is preferable that diffusion constants at 300° C. aresmaller than 10⁻²⁰ m²/s. According to FIG. 17 and FIG. 18, when thethickness of the capacitor insulating film is smaller than 35 nm, it ismore important to use ruthenium oxide or iridium oxide as an electrodematerial. In addition, even when 3 nm was retained as the thickness ofthe capacitor insulating film and the thickness of the electrode filmswas increased to 30 nm or 35 nm, there was obtained almost the sameresult as in FIG. 2. That is, even when the thickness of the electrodefilms is increased, in the case of using electrode materials other thanruthenium oxide and iridium oxide, diffusion constants at 300° C. become10⁻²⁰ m²/s or more.

According to the above-mentioned first to eighth examples, there can beprovided a thin film capacitor having high reliability. Furthermore,there can be provided a system-in-package having high reliability.

Hereinafter, other examples of the present invention will be describedin detail.

The plan layout of a semiconductor device according to the ninth workingembodiment of the present invention is shown in FIG. 20. FIG. 19 is thesectional view showing the sectional structure cut along the lineXIX-XIX of the semiconductor device shown in FIG. 20. In thesemiconductor device of the present working embodiment, as shown in FIG.19, element-separating films 302 consisting of, for example, siliconoxide films are provided at a certain interval on the surface of a Ptype silicon substrate 301, and an element-forming area 303 is formedbetween the element-separating films 302. On the element-forming area303, there is provided a P channel MOS transistor.

The MOS transistor is constituted by containing a gate insulating film1001 formed on the surface of the silicon substrate 301 and a gateelectrode 306 a facing the silicon substrate 301 through the gateinsulating film 1001. At both sides corresponding to saidelement-separating film sides of the gate electrode 306 a and gateinsulating film 1001, there are formed side walls 307 a consisting of,for example, silicon nitride. The gate insulating film 1001 isconstituted by containing at least a two layer-laminated structureconsisting of a titanium silicate film 304 a at the silicon substrateside and a titanium oxide film 305 a at the gate electrode film side.The gate electrode 306 a consists of, for example, a polycrystallinesilicon film, a metal thin film, a metal silicide film or the laminatedstructure thereof.

The MOS transistor shown in the figure has a P- type source-draindiffusion layer 308 formed in the state of self-aligning to the gateelectrode 306 a and a P+ type source-drain diffusion layer 309 formed inthe state of self-aligning to the element-separating film 302 and gateelectrode 306 a.

On the surface of this semiconductor device, there is formed aninterlaminar insulating film 310, and in this interlaminar insulatingfilm 310 there is provided a contact hole 311 leading to the P+ typesource-drain diffusion layer 309.

In order to satisfy the demand for miniaturization of transistors, thefactual thickness of the titanium silicate film 304 a is the one whichshould give the silicon-oxide equivalent thickness of the gateinsulating film 1001 of not more than 1 nm and which should preventincrease of leakage current. For example, when the silicon-oxideequivalent thickness of the gate insulating film 1001 is 1 nm and theelectric voltage applied to the gate insulating film is 1 V, the factualthickness of the titanium silicate film 304 a should be not less than1.0 nm but not more than 3.2 nm. Thereby, there can be obtained a gateinsulating film wherein leakage current is suppressed to a low level.

Next, a process for deriving the thickness of a titanium silicate filmeffective for suppressing increase of leakage current will be described.

FIG. 21 shows energy bands of the gate electrode, gate insulating film,and silicon substrate of the MOS transistor shown in FIG. 19. Herein,for example, the gate electrode consists of polycrystalline silicondoped with phosphorus. The gate insulating film consists of a two- layerstructure of titanium oxide film of thickness T₁ and titanium silicatefilm of thickness T₂, and the titanium silicate film is formed on thesilicon substrate side. In addition, the silicon substrate is a P typesubstrate. Ev, Ec, and Ef in the figure mean, respectively, valenceelectron band, conduction band, and Fermi energy of silicon. Φ_(B1) andΦ_(B2) mean energy barriers of titanium oxide and titanium silicate.

When relative dielectric constants of silicon oxide, titanium oxide, andtitanium silicate are respectively εSiO2, ε₁, and ε₂, silicon-oxideequivalent thicknesses T_(1eff), T_(2eff), and T_(eff) of said titaniumoxide film, titanium silicate film, and the gate insulating filmconsisting of the two layer-structure there of are respectivelyrepresented by the following Expressi

$\begin{matrix}{T_{1\quad{eff}} = {\frac{ɛ_{S\quad i\quad O\quad 2}}{ɛ_{1}}T_{1}}} & \left( {{Expression}\quad 1} \right) \\{T_{2\quad{eff}} = {\frac{ɛ_{S\quad i\quad O\quad 2}}{ɛ_{2}}T_{2}}} & \left( {{Expression}\quad 2} \right) \\{T_{eff} = {{T_{1\quad{eff}} + T_{2\quad{eff}}} = {ɛ_{S\quad i\quad O\quad 2}\left( {\frac{T_{1}}{ɛ_{1}} + \frac{T_{2}}{ɛ_{2}}} \right)}}} & \left( {{Expression}\quad 3} \right)\end{matrix}$

For example, when relative dielectric constants of silicon oxide,titanium oxide, and titanium silicate are shown by ε_(Si02)=4, ε₁=60,and ε₂=15, and when the film thicknesses are shown by T₁=15 nm, T₂=3 nm,and T=18 nm, the equivalent thicknesses become as shown by T_(1eff)=1nm, T_(2eff)=0.8 nm, and T_(eff)=1.8 nm.

FIG. 22 shows energy bands when a positive electric voltage V is appliedto the gate electrode. In this case, to the titanium oxide film and thetitanium silicate film there are appl and E_(ox2)

$\begin{matrix}{V_{1} = {\frac{ɛ_{2}T_{1}}{{ɛ_{2}T_{1}} + {ɛ_{1}T_{2}}}V}} & \left( {{Expression}\quad 4} \right) \\{V_{2} = {\frac{ɛ_{1}T_{2}}{{ɛ_{2}T_{1}} + {ɛ_{1}T_{2}}}V}} & \left( {{Expression}\quad 5} \right) \\{E_{o \times 1} = {V_{1}/T_{1}}} & \left( {{Expression}\quad 6} \right) \\{E_{o \times 2} = {V_{2}/T_{2}}} & \left( {{Expression}\quad 7} \right)\end{matrix}$

Tunnel electric current J flowing through the gate insulating filmconsisting of titanium oxide film and titanium silicate film shown abovecan be obtained by the following Expression 8 from the probability ofelectron's tunneling through the insulating film by use of WKB(Wentzel-Kramers-Brillouin) approach. $\begin{matrix}{{{J\left( {\Phi_{B},{T_{{o\quad x},}E_{o\quad x}}} \right)} = {\frac{n_{v}m_{d}k_{B}T}{2\quad\pi^{2}\hslash}{\int{T*{T_{WKB}\left( {\Phi_{B\quad 1},\Phi_{B\quad 2},T_{1},T_{2},E_{o\quad x\quad 1},E_{o\quad x\quad 2},E} \right)}I\quad{n\left( {1 + {\exp\left( \frac{E_{F} - E}{k_{B}T} \right)}} \right)}{dE}}}}}{{T*{T_{WKB}\left( {\Phi_{B\quad 1},\Phi_{B\quad 2},T_{1},T_{2},E_{o\quad x\quad 1},E_{o\quad x\quad 2},E} \right)}} = {\exp\left\{ {{A_{1}\left( {E_{n\quad 1} - E_{n\quad 2}} \right)} + {A_{2}\left( {E_{n\quad 3} - E_{n\quad 4}} \right)}} \right\}}}{A_{1} = \frac{4\sqrt{2\quad m_{i\quad n\quad s}}}{3\quad\overset{\_}{h}\quad q\quad E_{o\quad x\quad 1}}}{A_{2} = \frac{4\sqrt{2\quad m_{i\quad n\quad s}}}{3\quad\overset{\_}{h}\quad q\quad E_{o\quad x\quad 2}}}{E_{n\quad 1} = \left\{ {{\begin{matrix}\left\{ {\Phi_{B\quad 1} - \left( {E - E_{F}} \right)} \right\}^{3/2} & {E < {\Phi_{B\quad 1} + E_{F}}} \\0 & {E \geq {\Phi_{B\quad 1} + E_{F}}}\end{matrix}E_{n\quad 2}} = \left\{ {{\begin{matrix}\left\{ {\Phi_{B\quad 1} - \left( {E - E_{F}} \right) - V_{1}} \right\}^{3/2} & {E < {\Phi_{B\quad 1} + E_{F} - V_{1}}} \\0 & {E \geq {\Phi_{B\quad 1} + E_{F} - V_{1}}}\end{matrix}E_{n\quad 3}} = \left\{ {{\begin{matrix}\left\{ {\Phi_{B\quad 2} - \left( {E - E_{F}} \right) - V_{1}} \right\}^{3/2} & {E < {\Phi_{B\quad 2} + E_{F} - V_{1}}} \\0 & {E \geq {\Phi_{B\quad 2} + E_{F} - V_{1}}}\end{matrix}E_{n\quad 4}} = \left\{ \begin{matrix}\begin{Bmatrix}{\Phi_{B\quad 2} - \left( {E - E_{F}} \right) -} \\\left( {V_{1} + V_{2}} \right)\end{Bmatrix}^{3/2} & {E < {\Phi_{B\quad 2} + E_{F} - \left( {V_{1} + V_{2}} \right)}} \\0 & {E \geq {\Phi_{B\quad 2} + E_{F} - \left( {V_{1} + V_{2}} \right)}}\end{matrix} \right.} \right.} \right.} \right.}} & \left( {{Expression}\quad 8} \right)\end{matrix}$

In the above expressions,

-   n_(v): gate electrode electronic state degeneracy degree,-   m_(d): gate electrode electron effective mass,-   k_(B): Boltzmann constant,-   T: temperature,-   π: circular constant,-   h: Planck constant (in the expressions—is added to h),-   m_(ins): insulating film electron effective mass,-   E: energy of electrons, and-   E_(F): Fermi energy of gate electrode.

FIG. 23 shows dependency of leakage current density on the filmthickness T₂ and equivalent thickness T_(2eff) of titanium silicate,when the relative dielectric constant ε₂ of titanium silicate is 15,applied electric voltage is 1 V, temperature is 300 K, and theequivalent thickness T_(eff) of the gate insulating film is 1.0 nm. Thefigure shows results of calculation when the energy barrier Φ_(B2) oftitanium silicate is 1.5, 2.0, 2.5, and 3.0 eV. The leakage currentdensity is about 1.3×10⁻⁸ A/cm² when the gate insulating film consistsof only titanium oxide, that is, T₂=0 nm, and the leakage currentdensity decreases as the film thickness of titanium silicate increases.This is because a part of the electrons which can surpass the low energybarrier of titanium oxide cannot surpass the energy barrier of thesilicate.

The leakage current density shows the minimum value when the equivalentthickness of titanium silicate is about 0.7 nm and the factual filmthickness is about 2.5 nm, and the leakage current increases inaccordance with increase in the film thickness of titanium silicate.This is because electrons permeate the energy barrier made by titaniumsilicate through direct tunnels and tunnel current flows.

From FIG. 23, it is seen that the leakage current density changesdepending on the value of energy barrier of titanium silicate. However,when the electric voltage applied to the gate insulating film is 1 V andT₂ is 3.2 nm or less, it is seen that the leakage current can besuppressed to a lower value than the leakage current density in the casewhere the gate insulating film consists of only titanium oxide even whenthe value of energy barrier of titanium silicate changes.

Next, FIG. 24 shows dependency of leakage current density on the filmthickness T₂ and equivalent thickness T_(2eff) of titanium silicate,when the relative dielectric constant ε₂ of titanium silicate is 20, theelectric voltage applied to the gate insulating film is 1 V, and theequivalent thickness T_(eff) of the gate insulating film is 1.0 nm.Similarly to FIG. 23, the leakage current density decreases as thethickness of titanium silicate film increases from the point of T₂=0 nm,and the leakage current density shows the minimum value when theequivalent thickness of titanium silicate film is about 0.8 nm and thefactual film thickness is about 4.0 nm. Furthermore, if T₂ is 4.8 nm orless, it is seen that the leakage current can be suppressed to a lowervalue than the leakage current density in the case where the gateinsulating film consists of only titanium oxide even when the value ofenergy barrier of titanium silicate changes.

On the basis of the similar calculations, FIG. 25 and FIG. 26 showdependency of leakage current density on the film thickness T₂ andequivalent thickness T_(2eff) of titanium silicate, when the relativedielectric constant ε₂ of titanium silicate is 25 and 30. From thefigures, if the condition of the equivalent thickness 1 nm of gateinsulating film is satisfied by the film thickness of titanium silicate,it is seen that the leakage current can be suppressed to a lower valuethan the leakage current density in the case where the gate insulatingfilm consists of only titanium oxide, even when the value of energybarrier of titanium silicate changes, by providing a titanium silicatefilm.

Furthermore, in order to give good dielectric characteristics to atitanium silicate film, at least one lattice of thickness is consideredto be necessary, and hence factual thickness T₂ should be 1 nm or more.

From the above, even when the relative dielectric constant ε₂ oftitanium silicate is changed in the range of 15 to 30 and the energybarrier Φ_(B2) thereof is changed in the range of 1.5 eV to 3.0 eV, thevalue of leakage current flowing through the gate insulating film can besuppressed to a low value, by forming a titanium silicate film in afactual thickness T₂ of 1.0 nm to 3.2 nm.

Hereinabove, there was described the case where the silicon-oxideequivalent thickness of gate insulating film was 1 nm, the electricvoltage applied to gate insulating film was 1 V, and temperature was300° K. Also in the case of the other silicon-oxide equivalentthickness, electric voltage and temperature, the film thickness oftitanium silicate suitable for suppressing leakage current can bedecided by the similar process.

Next, in the case where the electric voltage applied to gate is 0.5 to 1V and the equivalent thickness is 0.7 to 1 nm, with regard to the filmthickness of titanium silicate suitable for suppressing leakage current,descriptions are given by use of FIG. 27, FIG. 28 and FIG. 29.

FIG. 27 shows dependency of leakage current density on the filmthickness T₂ and equivalent thickness T_(2eff) of titanium silicate,when the relative dielectric constant ε₂ of titanium silicate is 15,temperature is 300° K., and the equivalent thickness T_(eff) of the gateinsulating film is 1.0 nm. The figure is based on calculations in thecase where the applied electric voltage is 0.5 V, 0.7 V and 1 V and theenergy barrier ε_(B2) of titanium silicate is 1.5 eV.

As shown in FIG. 27, the leakage current density decreases as thethickness of titanium silicate film increases from the case where thegate insulating film consists of only titanium oxide, that is, T₂=0 nm.This is because a part of the electrons which can surpass the low energybarrier of titanium oxide cannot surpass the energy barrier made bytitanium silicate.

Furthermore, it is seen that the leakage current density shows theminimum value when the equivalent thickness of titanium silicate is 0.7nm and the factual film thickness is about 2.5 nm, and that the leakagecurrent increases in accordance with increase in the film thickness oftitanium silicate. This is because electrons permeate the energy barriermade by titanium silicate through direct tunnels and tunnel currentflows.

From FIG. 27, it is seen that the leakage current density changesdepending on the value of applied electric voltage. However, when theapplied electric voltage is in the range of 0.5 to 1 V, if T₂ is 3.2 nmor less, it is seen that the leakage current can be suppressed to alower value than the leakage current density in the case where the gateinsulating film consists of only titanium oxide.

In FIG. 27, there is shown the case where the energy barrier Φ_(B2) oftitanium silicate is 1.5 eV and the relative dielectric constant ε₂ is15. As stated above with reference to FIG. 23-FIG. 26, also in the casewhere the energy barrier Φ_(B2) is 1.5 to 3.0 eV and the relativedielectric constant ε₂ is 15 to 30, if T₂ is 3.2 nm or less, it can beshown that the leakage current can be suppressed to a lower value thanthe leakage current density in the case where the gate insulating filmconsists of only titanium oxide.

FIG. 28 shows dependency of leakage current density on the filmthickness T₂ and equivalent thickness T_(2eff) of titanium silicate,when the relative dielectric constant ε₂ of titanium silicate is 15,temperature is 300° K., and the equivalent thickness T_(eff) of the gateinsulating film is 0.7 nm. The figure is based on calculations in thecase where the applied electric voltage is 0.5 V, 0.7 V and 1 V and theenergy barrier Φ_(B2) of titanium silicate is 1.5 eV.

From FIG. 28, it is seen that the leakage current density changesdepending on the value of applied electric voltage. However, when theapplied electric voltage is in the range of 0.5 to 1 V and theequivalent thickness T_(eff) of the gate insulating film is 0.7 nm, ifthe film thickness T₂ of titanium silicate is 1.7 nm or less, it is seenthat the leakage current can be suppressed to a lower value than theleakage current density in the case where the gate insulating filmconsists of only titanium oxide.

In FIG. 28, there is shown the case where the energy barrier Φ_(B2) oftitanium silicate is 1.5 eV, the silicon-oxide equivalent thicknessT_(eff) of the gate insulating film is 0.7 nm and the relativedielectric constant ε₂ is 15. As stated above with reference to FIG.23-FIG. 26, also in the case where the energy barrier Φ_(B2) is 1.5 to3.0 eV and the relative dielectric constant ε₂ is 15 to 30, if T₂ is 1.7nm or less, it can be seen that the leakage current can be suppressed toa lower value than the leakage current density in the case where thegate insulating film consists of only titanium oxide.

For each thickness in the case where the silicon-oxide equivalentthickness of the gate insulating film is 0.7 to 1.0 nm, the range offactual film thickness of titanium silicate for suppressing increase ofleakage current can be obtained by the similar process. FIG. 29summarizes said desirable range of factual film thickness of titaniumsilicate corresponding to the silicon-oxide equivalent thickness 0.7-1.0nm of the gate insulating film in the case where electric voltageapplied to the gate is 0.5-1.0 V. The desirable range of factual filmthickness of titanium silicate shown in the figure corresponds to thecase where the relative dielectric constant of titanium silicate is 15,and in the case where the relative dielectric constant of titaniumsilicate is higher, a broader range can be obtained.

In addition, in the figure the factual film thickness T₂ of titaniumsilicate is indicated as 1.0 nm or more, and this is because at leastone lattice of thickness is necessary for giving titanium silicate gooddielectric characteristics.

The range of the factual film thickness T₂ of titanium silicate shown inFIG. 29 is represented by the following expression as a function of thesilicon-oxide equivalent thickness T_(eff) of the gate insulating film:1.0 (nm)≦T ₂≦5T _(eff)−1.8 (nm),wherein0.7 (nm)≦T _(eff)≦1.0 (nm).

That is, a semiconductor device having a gate insulating film whereinincrease of leakage current is suppressed can be obtained by forming,between titanium oxide and a silicon substrate, a titanium silicate filmhaving a thickness in the range of factual film thickness shown in FIG.29 in response to the silicon-oxide equivalent thickness Teff of thegate insulating film required by the specification of the semiconductordevice.

In the above working embodiment, there was described the case of apolycrystalline silicon film doped with phosphorus as a gate electrode.In addition to the polycrystalline silicon film, also in the case of agate electrode consisting of a metal thin film such as tungsten,molybdenum or the like, a metal compound such as tungsten nitride or thelike, or a metal silicide film such as tungsten silicide or the like, orthe laminated structure thereof, the film thickness of titanium silicateadequate for suppressing leakage current can be similarly decided bysuch process.

Because depletion does not occur in a gate electrode film formed of ametal film such as tungsten, molybdenum or the like, the equivalentthickness of a gate insulating film can be decreased. Furthermore,tungsten is thermally stable, and the film quality thereof is scarcelychanged in a high temperature process after formation of the gateelectrode film. In addition, when tungsten is laminated in contact withtitanium oxide, tungsten oxide is formed in some cases. Tungsten oxidehas a smaller dielectric constant than titanium oxide, and formation oftungsten oxide leads to the increase in the equivalent thickness of agate insulating film. Therefore, it is effective to use a tungstennitride or tungsten silicide film having excellent oxidation resistanceas compared with a tungsten film. Particularly in oxidation resistance,a tungsten nitride film is especially excellent. Moreover, when thetungsten nitride film is used for a gate electrode, by forming a gateelectrode 314 of a two-layer structure, wherein tungsten nitride 312 isused as a layer in contact with titanium oxide and tungsten 313 having alower resistance than tungsten nitride is used as the upper layer asshown in FIG. 30, a gate electrode having a low resistance can beobtained.

As stated above, according to the present working embodiment, titaniumsilicate film is present at the interface between titanium oxide filmand silicon substrate . Hence, a silicon oxide film having a lowrelative dielectric constant can be prevented from being formed at saidinterface and, at the same time, the silicon-oxide equivalent thicknesscan be decreased as compared with the case of providing silicon nitrideat said interface. Thus there can be provided a semiconductor devicehaving a gate insulating film which can satisfy miniaturization.

Furthermore, according to the present working embodiment, a gateinsulating film is constituted by a laminated structure of titaniumoxide film as a high dielectric constant material and titanium silicatefilm having a relatively large dielectric constant. Hence, while thefactual thickness of the gate insulating film can be made thick, thesilicon-oxide equivalent thickness can be made thin, and leakage currentcan be reduced.

Moreover, according to the present working embodiment, because there canbe obtained a semiconductor device wherein leakage current hardly flows,there can be obtained a semiconductor device having high reliability,and also there can be obtained a semiconductor device having a highyield.

The tenth example of the present invention is described by use of FIG.31 (including FIGS. 31(A)-31(C)), FIG. 32 (including FIGS. 32(A)-32(C))and FIG. 33 (including FIGS. 33(A)-33(C)). FIG. 31, FIG. 32 and FIG. 33show a process for preparing the semiconductor device having a gateinsulating film consisting of titanium oxide film and titanium silicatefilm shown in FIG. 19. Herein, there is described the case wherein thefactual thickness of titanium silicate film is 3 nm and the factual filmthickness of titanium oxide is 3 nm.

First, plural grooves of 200-300 nm in depth are formed at apredetermined interval on the surface of a P type silicon substrate 301,and silicon oxide films are embedded therein to form element-separatingfilms 302 of shallow groove type (FIG. 31 (A)).

Next, a titanium film 1010 of about 1 nm in thickness is formed on thesurface of the silicon substrate 301 by, for example, sputtering method(FIG. 31 (B)). Next, the titanium film 1010 is subjected to heattreatment of 600° C. to form titanium silicide film 1011. By thissilicide reaction, the thickness of titanium silicide film 1011 becomesabout 2 nm (FIG. 31 (C)). In addition, at this time, the portions incontact with element-separating films 302 are left as such without beingchanged into the silicide.

Next, the titanium silicide film loll is oxidized to form titaniumsilicate film 304 (FIG. 32 (A)). This oxidization reaction causes volumeexpansion, and the thickness of titanium silicate film 304 becomes about3 nm. When the thickness of titanium silicate film 304 is larger than 3nm, the titanium silicate film 304 is subjected to etching by sputteringmethod or the like to reduce the film thickness into a predeterminedthickness.

Next, a titanium oxide film 305 of about 3 nm in thickness is formed onthe surface of titanium silicate film 304 by, for example, CVD (ChemicalVapor Deposition) method.

Herein, when the equivalent thickness of a gate insulating film havingtitanium silicate film 304 and titanium oxide film 305 is larger than 1nm, the titanium oxide film 305 is subjected to etching by sputteringmethod or the like to reduce the film thickness into a predeterminedequivalent thickness.

Furthermore, a polycrystalline silicon film 306 containing impurephosphorus is formed on the surface of titanium oxide film 305 by CVDmethod or the like. The thickness of the polycrystalline silicon film306 is, for example, about 200 nm (FIG. 32 (B)).

Next, polycrystalline silicon film 306, titanium oxide film 305 andtitanium silicate film 304 are subjected to etching by use of aphotoresist film as a mask. Thereby, gate insulating film 1001 and gateelectrode 306 a of a MOS transistor are formed. Herein, the gateinsulating film 1001 has titanium silicate film 304 a and titanium oxidefilm 305 a (FIG. 32 (C)).

Next, there are formed P- type source-drain regions 308 of the MOStransistor by ion implantation of boron. The P- type source-drainregions 308 are in the state of self-aligning to the gate electrode andgate insulating film (FIG. 33 (A)).

Subsequently, a silicon nitride film 307 of 200 nm in thickness isdeposited on the surface of the semiconductor substrate by sputteringmethod or CVD method (FIG. 33 (B)), and the silicon nitride film 307 issubjected to etching to form side walls 307 a covering the side walls ofthe gate electrode and gate insulating film on element-separating film302 sides (FIG. 33 (C)).

Next, P+ type source-drain diffusion layers 309 are formed on thesilicon substrate 301 by ion implantation of boron with a mask ofelement-separating films 302, gate electrode 306 a and side walls 307 a.Subsequently, by CVD method there is formed an interlaminar insulatingfilm 310 covering element-separating films 302, gate electrode 306 a,side walls 307 a and P+ type source-drain diffusion layers 309, and inthe resultant interlaminar insulating film 310 there are formed contactholes 311 leading to the surfaces of P+ type source-drain diffusionlayers 309 from the surface thereof (see FIG. 19).

As stated above, first a titanium silicide film is formed on the surfaceof a silicon substrate, and then the titanium silicide film is oxidizedinto a titanium silicate film, on which a titanium oxide film is formed.This is because if the titanium oxide film is formed directly on thesurface of the silicon substrate, oxygen atoms in the titanium oxidefilm diffuse to the silicon substrate side, as stated previously, andsilicon oxide having a low dielectric constant is sometimes formed atthe interface between the titanium oxide film and the silicon substrate,which leads to a defective MOS transistor.

In the present working embodiment, a titanium silicide film is formed onthe surface of a silicon substrate, and then the titanium silicide filmis oxidized into a titanium silicate film, and hence the surface of thesilicon substrate is not contacted with oxygen atmosphere and there isno fear of formation of silicon oxide at the interface of the substrate.Furthermore, the titanium silicate film is formed on the surface of thesilicon substrate and a titanium oxide film is laminated on the silicatefilm, and hence oxygen atoms in the titanium oxide film are preventedfrom diffusing to the silicon substrate side. Moreover, the relativedielectric constant of titanium silicate is 15-40 whereas that ofsilicon nitride is about 7.8. Therefore, as compared with the case wherea silicon nitride film is formed at the interface between the titaniumoxide film and the silicon substrate, the factual thickness of titaniumsilicate film can be made larger than that of silicon nitride film, whenthe silicon-oxide equivalent thickness is the same. Therefore, theeffect of suppressing leakage current is large.

The above-mentioned preparation process relates to the case of a Pchannel MOS transistor. This preparation process is also applicable to aN channel MOS transistor, and, furthermore, is applicable, also, to aCMOS transistor and a BiCMOS transistor.

The eleventh example of the present invention is described by use ofFIG. 34 (including FIGS. 34(A)-34(D)). FIG. 34 shows some steps of aprocess for preparing the semiconductor device having a gate insulatingfilm consisting of titanium oxide film and titanium silicate film shownin FIG. 19. That is, FIGS. 34(A)-34(D) show the main process stepsleading to the formation of a titanium silicate film on a siliconsubstrate. Herein, there is described, for example, the case where thefactual thickness of titanium silicate is 3 nm and that of titaniumoxide is 3 nm.

First, plural grooves of 200-300 nm in depth are formed at apredetermined interval on the surface of a P type silicon substrate 301,and silicon oxide films are embedded therein to form element-separatinglayers 302 of shallow groove type (FIG. 34 (A)).

Next, a silicon oxide film 1020 of about 1.5 nm in thickness is formedon the surface of the silicon substrate 301 by, for example, thermaloxidation method (FIG. 34 (B)).

Furthermore, on the above silicon oxide film, there is formed a titaniumfilm 1021 of about 1.5 nm in thickness (FIG. 34 (C)).

Next, the above silicon oxide film 1020 and titanium film 1021 arereacted by heat treatment of 400° C. to 500° C. In this heat treatment,the silicon oxide film 1020 disappears and a titanium silicate film 304is formed by reduction reaction of titanium (FIG. 34 (D)). The thicknessof the above titanium silicate film 304 becomes about 3 nm, but when thethickness of titanium silicate film 304 is larger than 3 nm, thetitanium silicate film 304 is subjected to etching by sputtering methodor the like to reduce the film thickness and give a predeterminedthickness. When the thickness is smaller, it is possible to give apredetermined film thickness by adjusting the thicknesses of the abovesilicon oxide film and titanium film.

In the subsequent steps, a gate insulating film, a gate electrode filmand the like are formed to produce a MOS transistor similarly to FIG. 32(B) step and the subsequent steps of the above tenth example.

In the present working embodiment, a silicon oxide film 1020 is onceformed on a silicon substrate 301, but a titanium film 1021 is formed onthe silicon oxide film 1020 and the two films are reacted by heattreatment into a titanium silicate film 304. Hence, the silicon oxidefilm 1020 having a low dielectric constant disappears. Next, a titaniumoxide film is formed thereon and, therefore, at the time of forming thetitanium oxide film oxygen atoms in the titanium oxide film areprevented from diffusing to the silicon substrate side at the interfacewith the silicon substrate.

That is, also in the present working embodiment, there can be obtainedeffects similar to those of the above tenth example. In addition, asexamples of further preferred embodiments, the constitution concerningthe gate electrode structure containing the gate insulating film shownin the above ninth to eleventh example can be applied to thecorresponding portions in the first to eighth example.

According to the above ninth to eleventh examples, a titanium silicatefilm is present at the interface between titanium oxide film and siliconsubstrate. Hence, silicon oxide film having a low relative dielectricconstant can be prevented from being formed at said interface, and atthe same time the silicon-oxide equivalent thickness can be decreased ascompared with the case of providing silicon nitride at said interface,and thus there can be provided a semiconductor device having a gateinsulating film which can satisfy the desire for miniaturization.

Furthermore, a gate insulating film is constituted by a laminatedstructure of titanium oxide film as a high dielectric constant materialand titanium silicate film having a relatively large dielectricconstant. Hence, the factual thickness of the gate insulating film canbe made thick, the silicon-oxide equivalent thickness can be made thin,and leakage current can thereby be reduced.

Moreover, because there can be obtained a semiconductor device whereinleakage current hardly flows, there can be provided a semiconductordevice having high reliability, and also there can be provided asemiconductor device having a high yield.

The above descriptions were disclosed with reference to the variousexamples given. The present invention, however, should not be construedas being limited thereto. It should also be obvious to those skilled inthe art that various changes and modifications can be performed to thevarious disclosed and/or other alternative examples that are within thespirit and scope of the present invention and the attached claims.

INDUSTRIAL APPLICABILITY

The present invention relates to a semiconductor device and can beadapted to a semiconductor device having high reliability. Preferably,the present invention can be adapted to a semiconductor device providedwith a thin film capacitor having high reliability or a semiconductordevice having a gate structure wherein leakage current is suppressed.

1-9. (canceled)
 10. A semiconductor device comprising plural MOStransistors, each of which has a gate electrode formed through a gateinsulating film on a semiconductor substrate; wherein said gateinsulating film comprises a titanium silicate film formed on a surfaceof said semiconductor substrate and a titanium oxide film formed on thetitanium silicate film.
 11. A semiconductor device comprising plural MOStransistors formed, each of which has a gate insulating film presentbetween a semiconductor substrate and a gate electrode; wherein saidgate insulating film is constituted by a laminated structure containinga titanium silicate film formed on said semiconductor substrate side anda titanium oxide film formed on said gate electrode side.
 12. Thesemiconductor device of claim 10, wherein a silicon-oxide equivalentthickness of said gate insulating film, which is obtained fromdielectric characteristics, is not more than 1.0 nm.
 13. Thesemiconductor device of claim 10, wherein said titanium silicate filmhas a film thickness of not less than 1.0 nm but not more than 3.2 nm.14. The semiconductor device of claim 10, wherein a factual thickness,T₂, of said titanium silicate film is in a range represented by1.0 (nm)≦T ₂≦5T _(eff)−1.8 (nm) wherein T₂ represents the factualthickness of said titanium silicate film and T_(eff) represents thesilicon-oxide equivalent thickness of said gate insulating film. 15-18.(canceled)